Rapid ratiometric detection of Cd²⁺ based on the formation of ZnSe/CdS quantum dots

Developing simple and sensitive non-aggregation strategy for detecting Cd²⁺ is necessary for improving the selectivity and sensitivity of probe. Here, we establish a simple, rapid and ratiometric strategy for the recognition of Cd²⁺ based on the formation of core-shell ZnSe/CdS structure using ZnSe quantum dots (QDs). The transformation from binary ZnSe QDs to core-shell ZnSe/CdS QDs both change the elemental composition and structure of ZnSe QDs, leading to the changes in band gap of ZnSe QDs, which could be observed in the UV-vis spectra. In the detection process, ZnSe QDs only possess absorption peak at 343 nm, the formation of ZnSe/CdS after the addition of Cd²⁺ leads to the appearance of the new peak at 397 nm, while other heavy metal ions could not cause the appearance of new absorption peak. Therefore, this strategy shows good selectivity for Cd²⁺ detection. Based on this strategy, the limit of detection (LOD) for Cd²⁺ is 11 nM by UV-vis spectroscopy with a desirable relation of linearity (R² = 0.999) between A₃₉₇/A₃₄₃ and Cd²⁺ contents, which is superior to the LOD of most reported nanomaterials. The response time for Cd²⁺ detection is as short as 60 s, which is suitable for rapid detection. This ratiometric probe has also been applied to the detection of Cd²⁺ in tap water samples, the recovery of Cd²⁺ was between 94.9% and 105.6% for tap water samples, indicating the high accuracy of our ratiometric assay. Our strategy not only provided a new method for detecting Cd²⁺, but also put forward an implication that the band energy changes of QDs caused by heavy metal ions can be applied in the selective and sensitive detection of heavy metal ions.